Surely FB is not running MYSQL on a single machine. Perhaps i am misunderstanding what you are saying but saying SQL db's dont face the issues of distribution seems a little strange.
Distribution comes into picture from shape and size of the data not data saving/retrieval techniques. yea?
Also distributing reads is easy, writes... not so much. NoSQL systems usually offer distributed writes with the caveat of eventual consistency. RDBMS have referential integrity and other constraints which by definition cannot migrate into a distributed environment. Or at least there's not a one size fits all solution.
> Distribution comes into picture from shape and size of the data not data saving/retrieval techniques. yea?
Most definitely not. It has nothing to do with the shape and size of data. Also.. there's not such thing as "distribution" in our context. Only "distributed", from "distributed computing"[1] and it's everything to do about data saving and retrieval :)
so,
Use RDBMS if your data can be handled by a single machine( or have the resources of FB) ? '99% ppl need RDBMS' argument boils down to 99% of ppl have data that can be handled by a single machine RDBMS.
Is that a good conclusion?
If your application is like most apps(far more reads than writes) then you can easily distribute the load across multiple machines. If you have more writes than reads(quite rare but still) then scaling an RDBMS will be challenging.
In this case, if eventual consistency is something you can live with, a NoSQL store might be best for you.